(a) Field of the Invention
The present invention relates to a method for preparing a super absorbent polymer. More specifically, the present invention relates to a method for preparing a super absorbent polymer having a high fine powder aggregation strength.
(b) Description of the Related Art
Super absorbent polymer (SAP) is a synthetic polymer material having a function capable of absorbing moisture from about 500 to about 1,000 times of its own weight. Each manufacturer has denominated it as different names such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material) or the like. Such super absorbent polymers started to be practically applied in sanitary products, and now they have been widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultice or the like. As a method for preparing a super absorbent polymer, an inverse suspension polymerization method, an aqueous solution polymerization method or the like are known. For example, the inverse suspension polymerization is disclosed in Japanese Patent Publication Nos. Sho56-161408, Sho57-158209, Sho57-198714, and so on. Furthermore, the aqueous solution polymerization method, such as a thermal polymerization method in which a polymer gel is polymerized while being broken and cooled in a kneader equipped with a plurality of spindles, and a photopolymerization method in which an aqueous solution with a high concentration is irradiated with UV rays or the like onto a belt to carry out the polymerization and drying at the same time are known.
The hydrogel polymer obtained through said polymerization reaction is generally marketed as a powdered product after it is dried and pulverized.
At this time, fine powders (fines) having a particle diameter of about 180 μm or less may be formed during the steps of cutting, pulverizing and powdering the dried polymer. It is considered undesirable to apply the super absorbent polymer particle including the fine powder to hygiene goods such as a baby diapers and an adult urinary incontinence device because it may be moved before being used or may show decreased physical properties.
Therefore, the process for excluding the fine powders so that the fine powder is not included in a final product or the reassembling process for aggregating a fine powder to be normal particle size is needed. In this case, it is important to have a high aggregation strength so that the particles are not crushed again after the reassembling process. The reassembling process is generally carried out under a wet condition for raising the aggregation strength. At this time, the more moisture content of the fine powders, the more aggregation strength but it is not easy to handle the same during the reassembling process, and the less moisture content, the easier reassembling process but the aggregation strength is low and thus the fine powders may be easily crushed again after the reassembling.